Plasma generator



Dec. 8, 1964 R. A. DANDL ETAL PLASMA GENERATOR Filed Aug. 9. 1962 INN ' INVENTORS.

Raphael A. Dandl BY William B. Ard, Jr.

ATTORNEY.

. charge. 'very difiicult to obtain.

. drawing wherein: 1

represented by the United States Atomic Energy A Commission Filed Aug. 9, 1952, Ser. No. 215999 5 Claims. (Ql. 176-7) This invention relates to a method and apparatusfor producing a hot, stable plasma and for substantially reducing charge-exchange losses in such a plasma by providing a plasma blanket about the plasma by microwave heating of the electrons therein.

A plasma may be defined as a space charge neutralized ion gas and may be used in a variety of accelerators, mass spectrometers, high temperaturechemical reactors, and other electrical devices. Also, plasmas are desired for the study of the physics of plasmas, and in the experimentalstudy of controlled thermonuclear reactions.

in conventional practice, a plasma is highly contaminated with neutral gas particles. Collision processes of the neutral gas particles and other attendant phenomenon seriously alter the behavior of the plasma and may render such a plasma unstable or unfit for various uses. Power requirements in conventional methods of plasma production are high and plasma production eificiency is low. In conventional practice a plasma may be generated by ele tron bombardment of gas molecules as in an are dis- However, a stable, energetic plasma has been Thus, there exists a need for some method of substantially reducing the effects of neutral gas particles on such plasmas.

The present invention is for a method-and apparatus forproducing a stable, medium density, high temperature plasma in an evacuated enclosure permeated by a suitable magnetic field wherein the plasma is shielded from neutral gas particles by an energetic plasma blanket. This blanket is created by connecting a microwave generator to a reflecting cavity within the evacuated enclosure such that the electrons therein are heated at the electronground for dissociation, ionization, etc., for trapping, and

will provide'a means for optimizing the electron temperature for stability. A detailed description of the operating parameters for creating and sustaining such a plasma blanket will be described in detail below.

' I United States Patent 0 l It is therefore an object of this invention to provide a method and apparatus for producing an energetic stable plasma.

It is another object of this invention to provide a device for producing a plasma including means for shielding the plasma from neutral particles;

.It is still another object of this invention to provide a device wherein radio frequency energy is employed in the productionof ions ina plasma and for providing a plasma blanket about the plasma as a shield therefor.

Theseand other objects and advantages of this invert tlon will become apparent upon a' consideration-of the following detailed specification and the accompanying The single figure is a schematic out. 7 "In the drawing',j .a typical mirror field geometry is ing fa device I m which the principles of this invention may b can-ied i atented Dec. 8, 1964 portion may be about 12 inches, for example, and the length of this portion may be about 8 inches, for example. The overall length of the liner 4 on the axis thereof may be about 22 inches, for example. These dimensions are given by way of illustration only. It should be understood that the size of the liner 4 is not critical and may be made smaller or larger, if desired. It should be noted that the cavity defined by the liner 4 is of such size that it is many times larger than the fundamental wave length of the microwave energy, to be described below, which is fed into the cavity. Thus, the device is designed to operate in high modes. v

The liner 4 is provided with a Vacuum connection 5 and gas inletfee-d lines 6, 6. The gas feed lines communicate with portions of the liner 4 that project into the throat of the coils 2, 2. These ends of the liner 4 that project into the coils 2, 2 are provided with a plurality of microwave cut-off tubes '7, 7 to allow for plasma pumping and to prevent microwave energy from escaping through these ends of the liner 4. These tubes, '7, 7' have a diameter of about inch, for example. The liner 4 is connected to a microwave generator 3 through a waveguide transmission line 9. The end chambers 14% and 14-, as Well as the area surrounding the liner 5, and the chamber 15 are first evacuated to a pressure of 10- mm. Hg and then as feed gas is fed into the chamber 15 the pressure in chambers 14, 14 is raised to a selected pressure in the range of (3-6) X l0 mm. Hg, while the pressure in the chamber 15 within the liner dis raised to a selected value in the range from 1 10.- to 2X10 mm. Hg. The values of these pressures are set in accordance with the rate of gas feed into the chamber 15 within liner 4 and the value of the operating power of the generator 3. For example, when the generator 8 is operating at 3.5 kw. and 10.8 kmc. and the gas feed rate is .12 cc./sec., these pressures in chambers 14, 14' and 15 are adjusted to 6 l0 mm. Hg and 2 10 mm. Hg, respectively. When the generator 8 is operating at 1 kw. and 10.8 kmc. and the gasfeed rate is .06 cc./sec., these pressures in chambers 14, 14' and 15 are adjusted to 3 X l0 mm. Hg and 1 l0 mm. Hg, respectively. It has been determined that at an operating power of 3.5 kw. for the generator 8, the gas feed rate must not vary any measurable amount from 0.12 cc./se'c. in order to maintain a stable plasma, and when operated at the 1 kw. power level the feed rate must not vary any measurable amount from 0.06 cc./sec. for a stable plasma in the device described above. It should be noted that operation at the 3.5 kw. I

power level and a gas feed rate of 0.12 cc./sec. is the'most eflici nt in terms of ion density within the plasma. Thus. the device in the figure may be operated to produce a stable plasma with the microwave generator operating atany selected power from 1 to 3.5 kw. and at a frequency of 10.8 kmc. and the gas fee-d'rate is at some value from 0.06 cc./sec. to 0.12 cc./sec. depending on the operating of the coils and a flux density of'about 3410 gauss on the axis at the midpoint between the coils. The lines 13, 13 are lines of constant magnetic flux and the value of this flux is about 3820 gauss, for example, when the micro- 4.3 wave generator 8 is operating at 10.8 kmc. The magnetic field fiux is selected such that the cyclotron frequency for electrons within the liner 4 equals the frequency of the microwave input power.

In a typical operation of the device described above, feed gas which may be deuterium, for example, is fed at the desired, selected rate through the gas feed tubes 6, 6, and the pressures in the various chambers are adjusted to the desired values as discussed above. Simultaneously, the coils 2, 2' are energized to provide the above mentioned fiux densities and the microwave generator is set in operation at the desired power level from 1 to 3.5 kw. and at a frequency of 10.8 kmc. Under these operating conditions, a plasma 12 is produced by the energy supplied to the electrons in the system from the generator 8. Thus, heating at the electron cyclotron frequency is produced along the lines 13, 13' of constant magnetic flux. It should be noted that the liner 4 does not need to be resonant; rather, it needs only to be a reflecting cavity.

The conditions necessary for this heating process to exist are as follows. If one considers that initially a number of electrons are present with random velocities and random phases in their Larmor orbits, the condition that an appreciable number of them gain energy by a reflecting process is that they must be effectively brought into phase with the microwave electric field. The qualitative condition necessary for the electrons to be phased is that they gain an amount of energy between scattering electrons. The minimum electric field intensity needed becomes:

where 7\ has been replaced by 1/110. The number density of scattering centers is n, and is the effective cross section for a 90 scattering. If this condition is satisfied, an electron with initial energy E, and 180 out of phase with the applied microwave field will lose an amount of energy 5,, and then gain energy because it Will be in phase with the field for the remainder of its free path. Electrons less than 180 out of phase with the microwave field will gain more energy, and electrons starting in phase with this field will have an energy greater than 2E at the end of a mean scattering time. The average energy of the electrons after one scattering time will thus be greater than 1-3,. The energy gained per cycle is proportional to the Larmor radius of the electron and thus the rate of energy gain increases with increasing electron energy.

due to the ionization of the residual neutral gas within the liner 4. As these ions gain energy from the hot electrons or auxiliary ion heating processes they will be confined within the area Till inside of the plasma 12. The deute-ri um gas being fed into the chamber 15 through tubes 6, 6' is also ionized by the heated electrons. It has been determined that for a given gas feed rate and for a selected operating power for the microwave generator 8, that the ions produced by the ionization of this feed gas will form a plasma blanket with a thickness T. The electron density and thickness of this blanket are a function of the operating power of the generator 8 and are greatest at the 3.5 kw. operating level. As mentioned above the gas feed rate is critical for each operating power level in order to produce a stable plasma, and at 3.5 kw. the critical gas feed rate is 0.12 cc./sec. This blanket will not form when the feed rate is higher because the relatively cold gas particles in the gas feed will prevent such a formation, and when the gas feed rate is lower the plasma is unstable and no blanket formation will occur.

The plasma blanket thus formed acts as a shield against neutral particles trying to enter the plasma from the area surrounding the plasma blanket. Any neutral particle entering this blanket will quickly become ionized before it can reach the plasma confined within the area 11 shielded by this blanket. Thus, the plasma in this area it is essentially protected against charge exchange losses by the plasma blanket. The blanket provides a background for dissociation, ionization and for trapping, and provides a means for optimizing the electron temperature for stable operating conditions. Under the above operating conditions the electron density in the blanket is about 2 10 electrons/cm and these electrons have a temperature equivalent to about 20 kv. Such a blanket will cause the density 12 of neutral particles of mean thermal energy in the central plasma in the area 11 to decrease according to the formula r1 n r where n is the neutral density outside the blanket plasma, e is the microwave field, and

where T is the thickness of the blanket and A is the mean free path of the electrons for ionization of the background gas. In a typical operation of the above device, A is about 2.5 cm. Thus, the neutral particles outside the plasma blanket are effectively prevented from reaching the area 11 within the blanket since they become ionized before they ever reach this area and the number of neutral particles in the area 11 are reduced in accordance with the above formula. Therefore, a stable, dense and very energetic plasma is formed and maintained in the area 11. It has been determined that at the 3.5 kw. operating power level the ion density of the central plasma under stable operating conditions is at some value from 5 10 to 10 ions/cm. At the 1 kw. operating level this ion density ranges from (2.53) l0 ion/cm. Thus, it can be seen that as the operating power level is increased the ion density will increase.

The ion density of the trapped plasma may be increased further by the injection of a beam of neutral tritium directly into the plasma blanket, where it is quickly ionized by the hot electrons within this blanket. The neutral tritium beam may have an injection energy of a selected value in the range from 50-l00 kev., for example.

It should be noted that the effectiveness of the plasma blanket in stopping neutral particles can easily be determined by directing a beam of neutral particles toward such a blanket and observing if any of these neutrals actually penetrate and can be detected on the other side of such a blanket. The neutrals would have to actually pass through the blanket twice in any passage through the central area 11 in the above described device. It has been determined that the plasma blanket is very effective in stopping such a neutral beam.

A good indication of a stable plasma is when simultaneously the X-rays from the plasma have a constant measured value, the electron temperature has a constant average measured value, and the diamagnetic properties of theplasrna are stable. Diagnostic measurements of the plasma produced in the operation of the above described device have clearly indicated that a stable plasma is providedunder the specified operating conditions.

arouses In the operation of the above described device, it has been observed and confirmed by measurements that some neutrons have been produced. It is assumed that the energy and density of the ions in the stable, hot plasma have acquired snificient values for the production of such neutrons. Thus the device described above can also be used as a neutron source.

It should be noted that the present device is not limited for use with deuterium only as the feed gas. A mixture of deuterium and tritium can also be used as the feed gas, if desired. Also, the present device can be operated with a microwave generator which provides a shorter wave length than that described above. With such a generator, the size of the blanket plasma can be reduced substantially and the magnetic flux can be increased, thus increasing the energy-density of the inner plasma. In addition, when the device is operated with a shorter wave length, it may be feasible to dispense with the reflecting cavity defined by the liner 4, and the walls 1 of the outer chamber are then used as the reflecting cavity.

This invention has been described by way of illustration rather than limitation and it should be apparent that the invention is equally applicable in fields other than those described.

What is claimed is:

1. A method of producing a stable, energetic, dense plasma within an evacuated reflecting cavity disposed within an evacuated enclosure permeated by axially symmetric magnetic mirror fields comprising the steps of evacuating said cavity and said enclosure to a first selected pressure, feeding high frequency microwave energy of a selected power and frequency into said evacuated reflecting cavity, establishing said magnetic fields at a strength such that the electron cyclotron frequency is made equal to the frequency of said microwave energy, the electrons within said cavity being heated by said microwave energy along lines of constant magnetic flux provided by said magnetic fields, said heated electrons ionizing the background gas within said cavity to form a primary plasma, feeding gas at a selected critical rate into said cavity and simultaneously raising the pressure within said enclosure and cavity to second and third pressures, respectively, said critical gas feed rate being proportional to the selected operating power of said microwave energy for any selected cavity size, whereby a stable plasma blanket is formed surrounding said primary plasma, said blanket shielding said primary plasma from neutral particles and rendering said primary plasma stable, energetic and dense.

2. The method'set forth in claim 1, wherein said first pressure is mm. Hg, said second pressure in said enclosure is of a selected value in the range from (3-6) X 10- mm. Hg, said third pressure in said cavity is of a selected value in the range from (1-2) 10 mm. Hg, said gas feed rate is at a selected value in the range from 0,06 cc./sec. to 0.12 cc./sec., said microwave energy is of a selected power setting from 1 to 3.5 kw. and at an operating frequency of 10.8 kmc., and said magnetic mirror coils having a mirror ratio of 2: 1, said lines of constant magnetic flux having a selected value of about 3820 gauss.

3. The method set forth in claim 1,wherein said selected gas feed rate is 0.12 'cc./sec., said second, selected enclosure pressure is 6 10- mm. Hg, said third, selected cavity pressure is 2X l0- mm. Hg, said selected operating power for said microwave energy is 3.5 kw., and said feed gas is deuterium, whereby said stable primary plasma has an ion density of about 10 ions per cubic centimeter.

4. The method set forth in claim 2, wherein said selected gas feed rate is 0.06 cc./sec., said second, selected enclosure pressure is 3 X 10* mm. Hg, said third, selected cavity pressure is 1 10- mm. Hg, said selected operating power for said microwave energy is 1 kw., and said feed gas is deuterium, whereby said stable primary plasma has an ion density of about 3 X 10 ions per cubic centimeter.

5. A method of producing a stable, energetic, dense plasma within an evacuated enclosure permeated by aXially symmetric magnetic mirror fields, comprising the steps of evacuating said enclosure to a first selected pressure, feeding high frequency micromave energy of a selected power and frequency into said evacuated enclosure which acts as a reflecting cavity, establishing said magnetic fields at a strength such that the electron cyclotron frequency is made equal to the frequency of said microwave energy, the electrons within said enclosure being heated by said microwave energy along lines of constant magnetic flux provided by said magnetic fields, said heated electrons ionizing the background gas within said enclosure to form a primary plasma, feeding gas at a selected critical rate into said enclosure and simultaneously raising the pressure within said enclosure to a selected second pressure, said critical gas feed rate being proportional to the selected operating power level of said microwave energy for any selected enclosure size, whereby a stable plasma blanket is formed surrounding said primary plasma, said blanket shielding said primary plasma from neutral particles and rendering said primary plasma stable, energetic and dense.

References Cited by the Examiner UNITED STATES PATENTS 3,015,618 1/62 Stix 176-3 X 3,022,236 2/62 Ulrich et al. l761 3,031,399 4/62 Warnecke et al 176-2 3,032,490 5/62 Simon 176-7 3,105,803 10/63 Weibel 176'1 3,113,088 12/63 Josephson 1761 CARL D. QUARFORTH, Primary Examiner.

REUBEN EPSTEIN, Examiner. 

1. A METHOD OF PRODUCING A STABLE, ENERGETIC, DENSE PLASMA WITHIN AN EVACUATED REFLECTING CAVITY DISPOSED WITHIN AN EVACUATED ENCLOSURE PERMEATED BY AXIALLY SYMMETRIC MAGNETIC MIRROR FIELDS COMPRISING THE STEPS OF EVACUATING SAID CAVITY AND SAID ENCLOSURE TO A FIRST SELECTED PRESSURE, FEEDING HIGH FREQUENCY MICROWAVE ENERGY OF A SELECTED POWER AND FREQUENCY INTO SAID EVACUATED REFLECTING CAVITY, ESTABLISHING SAID MAGNETIC FIELDS AT A STRENGTH SUCH THAT THE ELECTRON CYCLOTRON FREQUENCY IS MADE EQUAL TO THE FREQUENCY OF SAID MICROWVER ENGERY, THE ELECTRONS WITHIN SAID CAVITY BEING HEATED BY SAID MICROWAVE ENERGY ALONG LINES OF CONSTANT MAGNETIC FLUX PROVIDED BY SAID MAGNETIC FIELDS, SAID HEATED ELECTRONS IONIZING THE BACKGROUND GAS WITHIN SAID CAVITY TO FORM A PRIMARY PLASMA, FEEDING GAS AT A SELECTED CRITICAL RATE INTO SAID CAVITY AND SIMULTANEOUSLY RAISING THE PRESSURE WITHIN SAID ENCLOSURE AND CAVITY TO SECOND AND THIRD PRESSURES, RESPECTIVELY, SAID CRITICAL GAS FEED RATE BEING PROPORTIONAL TO THE SELECTED OPERATING POWER OF SAID MICROWAVE ENERGY FOR ANY SELECTED CAVITY SIZE, WHEREBY A STABLE PLASMA BLANKET IS FORMED SURROUNDING SAID PRIMARY PLASMA, SAID BLANKET SHIELDING SAID PRIMARY PLASMA FROM NETURAL PARTICLES AND RENDERING SAID PRIMARY PLASMA STABLE, ENERGETIC AND DENSE. 